1. Field of the Invention
The present invention relates to the fields of nuclear magnetic resonance spectroscopy and magnetic resonance (MR) imaging, and more specifically to the field of longitudinal spin relaxation time measurement.
2. Discussion of Prior Art
Presently, the measurement of spin-lattice relaxation times is a useful procedure in the fields of both high-resolution Nuclear Magnetic Resonance (NMR) spectroscopy and in Magnetic Resonance Imaging. In these procedures a sample (or subject) is placed in a magnet causing resonating nuclei of the subject, or "nuclear spins", to generate longitudinal spin magnetization. In a common procedure, this magnetization is inverted by the application of a radio frequency pulse to the subject capable of nutating the longitudinal spin magnetization 180.degree.. When the magnetization of the sample's nuclear spins is inverted, it spontaneously returns to the non-inverted equilibrium state. The return to the equilibrium state occurs in an exponential fashion having a half-life which is characteristic of the molecular environment of the nuclear spin. This half-life is frequently given the name longitudinal spin relaxation time, T.sub.1.
During the return to the equilibrium (or fully relaxed) state, the longitudinal magnetization cannot be directly detected. The instantaneous amount of longitudinal magnetization can be measured, however, by applying a sampling radio frequency (RF) pulse. This sampling RF pulse nutates the longitudinal magnetization into the transverse plane, thereby creating transverse spin magnetization. Maximum transverse spin magnetization is generated by the application of a 90.degree. nutation. Unlike longitudinal magnetization, transverse spin magnetization is capable of inducing a signal in a receiver coil placed near the sample.
The signal induced in a receiver coil carries significant information about the local environment of signal generating nuclei. If the signal is acquired in a homogeneous magnetic field, the spectral components of the signal can be resolved to provide a nuclear magnetic spectrum in which different peaks arise from populations of nuclei in different molecules (or parts of a molecule). The T.sub.1 of individual peaks can vary considerably across a spectrum and can provide useful analytical information about molecular structure.
If the spatial distribution of transverse spin magnetization is to be measured (as in MRI), the transverse spin magnetization can be phase shifted using magnetic field gradient pulses of selected intensities and durations. These gradient-induced phase shifts encode the position of spin magnetization within the magnet. Two or three-dimensional images of the distribution of spin magnetization can be generated by repeating the sequence of RF and magnetic field gradient pulses and acquiring the MR signal responsive to a collection of magnetic field gradient intensities.
Measurement of T.sub.1 with previously available methods in NMR spectroscopy and MR imaging typically requires a long acquisition time. This is because the longitudinal magnetization must be measured at multiple points in time after the inversion pulse to accurately determine the half-life of the recovery. Only a single sampling pulse can be used during the recovery process. This is because application of a sampling pulse disturbs the longitudinal spin magnetization, and thus compromises the integrity of measurements generated by any subsequent sampling pulses. Furthermore, best results are obtained when full recovery of longitudinal spin magnetization occurs after each sampling pulse. For in-vivo applications the time for full relaxation is between 1500 and 5000 ms, since most in-vivo T.sub.1 values are between 300 and 1000 ms. Measurement of T.sub.1 for each pixel in an image may require exam times as long as an hour, since enough data must be acquired to construct an image (typically with a resolution of 256.times.256), for each of several sampling times (typically 4-8) after each inversion pulse.
An alternative method for in-vivo T.sub.1 measurement described by Campeau et. al. in the Proceedings of the Eleventh Annual Meeting of the Society of Magnetic Resonance in Medicine, 1992, pg. 434, employs a series of slice selective inversion pulses which excite slices placed orthogonal to the image plane of an acquired MR image. Each inverted slice is in a unique location and each inversion pulse is applied at a unique time before the application of the transverse spin magnetization generation pulse of the imaging pulse sequence. If the acquired image has relatively large features of homogeneous T.sub.1, (e.g. a large skeletal muscle) the resulting image will contain a series of stripes, each created by spin inversion at selected times prior to the application of the detection pulse. The T.sub.1 values of the selected image feature can then be determined by measuring the pixel intensity in each stripe corresponding to each inversion delay time and fitting the result to an exponential equation to determine the rate constant, T.sub.1. While this method is relatively fast, it is not suited for the T.sub.1 measurement of small features such as the blood in a selected blood vessel. The technique is also poorly suited for T.sub.1 measurement of moving blood, since blood motion during the period between each selective inversion pulse and the detection pulse causes mixing of the inverted boluses of blood.